ScGaN alloy growth by molecular beam epitaxy: Evidence for a metastable layered hexagonal phase

Alloy formation in ScGaN is explored using rf molecular beam epitaxy over the Sc fraction range x=0-100%. Optical and structural analysis show separate regimes of growth, namely (I) wurtzitelike but having local lattice distortions in the vicinity of the Sc-Ga substitutions for small x (xless than or equal to0.17), (II) a transitional regime for intermediate x, and (III) cubic, rocksaltlike for large x(xgreater than or equal to0.54). In regimes I and III, the direct optical transition decreases approximately linearly with increasing x but with an offset over region II. Importantly, it is found that for regime I, an anisotropic lattice expansion occurs with increasing x in which a increases much more than c. These observations support the prediction of Farrer and Bellaiche [Phys. Rev. B 66, 201203-1 (2002)] of a metastable layered hexagonal phase of ScN, denoted h-ScN.